Evaluation of Modified Boehm Titration Methods for Use with Lignocellulosic Biochars.

The Boehm titration, originally developed to quantify functional groups of carbon blacks and activated carbons in discrete pK_a ranges, has earned growing attention for analyzing biochar (Boehm, 1994; Goertzen et al., 2010). However, properties that distinguish biochar from carbon black and activated carbon, including greater carbon solubility and higher ash content, may render the original Boehm titration method unreliable for use with biochars. Here we use seven biochars and one carbon black to evaluate three modified Boehm titration methods which employ (1) acidification followed by sparging, (2) centrifugation with barium chloride, and (3) use of a solid phase extraction cartridge followed by acidification and sparging to remove carbonates and dissolved organic compounds (DOC) from the Boehm extracts prior to titration. Our results for the various combinations of Boehm reactants and methods indicate that no one method was free of bias for all three Boehm reactants. Instead, we recommend a combination of the sparging method without a cartridge for quantifying functional groups in the lowest pK_a range (~5-6.4), and the barium centrifugation method for quantifying functional groups in the higher pK_a ranges (~6.4-10.3 and ~10.3-13).